In recent years, semiconductor devices have become more integrated, and structures of semiconductor elements have become more complicated. Further, the number of layers in multilayer interconnections used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device have increased, so that step heights on the surface of the semiconductor device tend to be larger. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated many times to form subsequent thin films on the semiconductor device.
When the number of irregularities is increased on a surface of a semiconductor device, the following problems arise. The thickness of a film formed in a portion having a step is relatively small when a thin film is formed on a semiconductor device. An open circuit is caused by disconnection of interconnections, or a short circuit is caused by insufficient insulation between interconnection layers. As a result, good products cannot be obtained, and the yield tends to be reduced. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after long-term use. At the time of exposure in a lithography process, if the irradiation surface has irregularities, then a lens unit in an exposure system is locally unfocused. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it is difficult to form a fine pattern itself on the semiconductor device.
Accordingly, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. The most important one of the planarizing technologies is CMP (Chemical Mechanical Polishing). In the chemical mechanical polishing, with use of a polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, which is referred to as a top ring or a carrier head, for holding a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing table under a predetermined pressure by the substrate holding apparatus. At this time, the polishing table and the substrate holding apparatus are moved relatively to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may insufficiently be polished or may excessively be polished at some portions depending on the pressing force applied to those portions of the semiconductor wafer. Therefore, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding apparatus by an elastic membrane made of an elastic material such as rubber and to supply fluid pressure such as air pressure to a backside surface of the elastic membrane to unformize pressing forces applied to the semiconductor wafer over an entire surface of the semiconductor wafer.
Further, the polishing pad is so elastic that pressing forces applied to a peripheral portion of the semiconductor wafer being polished become non-uniform, and hence only the peripheral portion of the semiconductor wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, there has been used a substrate holding apparatus in which a semiconductor wafer is held at its peripheral portion by a guide ring or a retainer ring, and the annular portion of the polishing surface that corresponds to the peripheral portion of the semiconductor wafer is pressed by the guide ring or retainer ring.
The thickness of a thin film formed on a surface of a semiconductor wafer varies from position to position in a radial direction of the semiconductor wafer depending on the film deposition method or the characteristics of a film deposition apparatus. Specifically, the thin film has a film thickness distribution in the radial direction of the semiconductor wafer. Since a conventional substrate holding apparatus, as described above, for uniformly pressing an entire surface of a semiconductor wafer polishes the semiconductor wafer uniformly over the entire surface thereof, it cannot realize a polishing amount distribution that is equal to the aforementioned film thickness distribution on the surface of the semiconductor wafer.
There has been proposed a polishing apparatus for applying locally different pressures to a semiconductor wafer to make the pressing force for pressing a thicker film region on the semiconductor wafer against a polishing surface greater than the pressing force for pressing a thinner film region on the semiconductor wafer against the polishing surface, thereby selectively increasing the polishing rate of the thicker film region. Thus, the overall surface of the substrate can be polished uniformly irrespective of the film thickness distribution that has been provided when the film is grown on the semiconductor wafer.
However, when the pressures of a fluid such as pressurized air supplied to respective pressure chambers positioned on the reverse side of the semiconductor wafer are independently controlled to locally control the pressure applied to the semiconductor wafer to press the semiconductor wafer under locally different pressures, if a membrane (elastic membrane) that is present over the boundary between different pressures is damaged or if airtightness is not kept between the semiconductor wafer and the membrane to cause an air leakage, then the various regions of the semiconductor wafer cannot be pressed under predetermined pressing forces, thus adversely affecting the result of the polishing process. When an air leakage occurs, then the regulator which regulates the pressure of the fluid increases the flow rate of the fluid in order to make up for a pressure drop caused by the air leakage. Therefore, it is difficult to detect a slight air leakage by monitoring fluid pressures. If a single flow meter is used to detect an air leakage, such detection method suffers detection errors frequently and is not reliable enough because a flow of fluid is generated even when the fluid is pressurized in a non-leakage state.